All species exhibit daily cycles in physiology and behavior called circadian rhythms. For example, in humans core body temperature oscillates between an afternoon high and a nighttime low. Circadian rhythms are generated by a pacemaker located in the hypothalamus. In isolation from external time cues (zeitgebers), the intrinsic period of this pacemaker is about 25 hours. Ordinarily the pacemaker responds to zeitgebers in such a way that it becomes entrained to the 24-hour solar day. In most species light (dawn, dusk) is the most important zeitgeber. The phase response to light of the circadian pacemaker is such that (1) its period becomes 24 hours and (2) it adopts a characteristic phase position relative to the zeitgeber (e.g., sleep occurs once every 24 hours and principally at night). The effect of light pulses administered to animals free-running in constant conditions depends on when in the circadian cycle the pulse is presented. Pulses near subjective morning produce phase advances, near subjective evening delays, and during subjective daytime no effect. These responses can be used to generate a phase-response curve which shows direction and magnitude of phase shift as a function of subjective time of pulse presentation. The purpose of this project is to generate a phase response curve to light for the human circadian system. Using suppression of pineal melatonin secretion as a marker, we previously found that the human hypothalamus is relatively insensitive to light. Healthy subjects living for two weeks in constant dim light are exposed to a single 6-hour pulse of bright artificial light (3000 lux) administered at different subjective times in different subjects. Circadian rhythms are monitored longitudinally in rectal temperature, wrist motor activity, sleep EEG, and behavioral events. Shifts in these rhythms induced by light are recorded. The normal human phase response curve is expected to prove valuable in understanding and treating circadian rhythm disturbances in depression, mania, insomnia (delayed sleep phase syndrome), jet lag and shift work.